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STRUCTURAL 

DESIGN 

OF 

REFRACTORY 
CONCRETE 



BY 

R. T. GILES 

The Atlas Lumnite Cement Co 

New York, N. Y. 



Structural Design of Refractory Concrete 



By R. T. GILES 

The Atlas Lumnite Cement Co 
New York, N. Y. 



REFRACTORY concrete differs 
■^ from structural concrete in 
that a calcium aluminate cement 
and refractory aggregates are used 
instead of portland cement, sand 
and gravel or stone commonly used 
in structural concrete. Refractory 
concrete is used for continuous ex- 
posure to temperature as high as 
3000 deg. F. while structural con- 
crete is usually limited to continu- 
ous exposure of temperatures no 
higher than 500 deg. F. to 600 
deg. F. 

Very little is known about struc- 
tural design of refractory concrete 
at this time. Only recently have 
data been available on the ultimate 
unit strength of the concrete after 
it has been exposed to elevated 
temperatures over long periods of 



•"Inyestigation of Certain Prop- 
er' of Uefractory Concrete," Bul- 
letin, American Ceramic Society, 
Sept. 1939. 



time.* Plots of these data are 
shown in Figs. 1 and 2. Refrac- 
tory concrete heated only on one 
side differs from structural con- 
crete, in that at temperatures 
above 1500 deg. F. it has a higher 
strength at the inside face and 
outside face than it has in the 
middle of the mass. The strength 
at the inside face is a result of its 
exposure to elevated temperatures. 
The strength at the outside face 
exposed to the air is similar to 
that of regular concrete. At some 
point in the inner part of the con- 
crete there is a weak section or 
plane similar to a very low quality 
concrete. To use any one of these 
strengths individually as a unit 
design strength would be mislead- 
ing. It is necessary, therefore, to 
use the average strength of the 
concrete for design purposes. 

Fig. 3 shows the strengths of 
the concrete which can be expected 
if a temperature gradient through 



c 

■ 

<n 



2000 



1800 



1600 



1400 



1200 



10OO 



1 1 r~ 

Dotted Line . 

Clay Added to High Vitrification Cloy Aggregates 

Solid Line = High Vitrification Clay Aggregatea. 



T 1 1 1 1 1 1 1 1 r 

Low Vitrification Clay Aggregate or 



1 1 1 1 

Low Vitrification 




200 



400 



600 



800 



1000 



1200 



1400 



1600 



1800 2000 



2200 



2400 



Temperature, Degree* F. 



Fig. 1 The average ultimate strength of concrete slabs or walls exposed to elevated temperatures 
for long periods. These results are applicable to concrete with cold compressive strength of 2000 lb. 
per sq. In. or more and with cold Rexural strength of 500 lb. per sq. in. or more when using calcium 
aluminate cement as the binder and a strong aggregate in the proportions of 1 bag to 4 cu. ft. of 
aggregate. 



1000 



C 800 








200 



400 



600 



800 1000 1200 1400 
Temperature, Degrees F. 



1600 



1800 2000 



Fin 2 Ultimate compressive and flexural strength which may be used for determining the average 
ultima UsLg of concrete slabs or walls exposed to elevated temperatures over long period of ^ time. 
ThKP results are aoolicable to concrete w th cold compressive strengths of 1000 lb. per sq. in. or less 
when using a calcium aluminate cement and a weak insulating aggregate in the proport.ons of one bag 
of cement to 4 cu. ft. of aggregate. 



the wall is plotted and strength of 
each section used to calculate the 
average unit strength. 

Example: Using a furnace wall 
of 6" thick exposed to 2100 deg. F. 
on the hot face with 300 deg. F. 
on the cold face. This gives a 
temperature drop of 1800 deg. F. 
or 600 deg. F. for each 2" section. 
The average temperature in the 
section containing the hot face is 
1800 deg. F. The average temper- 
ature of the middle section is 1200 
deg. F. and the average tempera- 
ture of the section containing the 
cold section is 600 deg. F. By re- 
ferring to the design chart, Fig. 1, 
we find the compressive strength 
using high vitrification aggregate 
at 1800 deg. F. to be 500 lbs. per 

compressive strength 

F. to be 600 lbs. per 

compressive strength 

F. to be 1200 lbs. per 



sq. 

at 

sq. 



in. ; the 
1200 deg. 

in.; the 
600 deg. 



at 

sq. in. By adding 1200 + 600 + 500 

and dividing by 3, the number of 
sections, we have 766 lbs. per sq. 
in. for the average compressive 
strength for the concrete in the 
wall. 

Example: Using a 12" wall ex- 
posed to 2700 deg. F. on the hot 
face with a cold face temperature 
of 300 deg. F. (See Fig. 3) This 
give- a temperature drop of 2400 
deg. F. or 400 deg. F. for each 2" 
section. The average temperature 
in the first J section is 2500 deg. 



F. which has lbs. per sq. in. com- 
pressive strength. The next sec- 
tion has an average temperature 
of 2100 deg. F. which has a com- 
pressive strength of 600 lbs. per 
sq. in. The next section has an 
average temperature of 1700 deg. 
F. which has a compressive 
strength of 450 lbs. per sq. in.; the 
next a temperature of 1300 deg. 
F. and a compressive strength of 
500 lbs. per sq. in. ; the next a tem- 
perature of 900 deg. F. with a 
compressive strength of 850 lbs. 
per sq. in., and the outside section 
an average temperature of "500 
deg. F. with a compressive strength 

of 1350 lbs. per sq. in. By adding 
+ 600 + 450+500 + 850+1350, and 

dividing by 6, we have 625 as the 
average compressive strength in 
lbs. per sq. in. for the 12" thick- 
ness. 

Of course, it is realized that the 
average strength is not an exact 
measure of the effective strength 
of a wall whose elements have 
varying unit strengths. Formulas 
for exact strength can be integral 
ed by means of the Calculus, but 
their application is laborious. In 
addition, consideration should be 
given to any difference in modulus 
of elasticity of the concrete at dif- 
ferent temperatures. Assuming 
that these moduli do not differ ap- 
preciably, a simple approximation, 



4 



Temperature, Degrees F 



3000 



2800 



o 

o 

to 



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o 



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o 



o 
o 



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o 

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2600 



2400 



2200 



2000 



1800 



1600 



Temperature Gradient 
of 12" Wall 



oo 1400 



0) 



s 



1200 



1000 



Strength Gradient from 
Design Chart No. 1 




Arbitrary Strength Gradient 






800 



600 



400 




Cold Face 
300 Dee. F. 







2 



4 



11 



Inche s 

Fig. 3 Wall strength of refractory concrete. Broken line: Strength gradient of refractory concrete 
wall 12 in. thick exposed to 2700 (leg. on hot face; outside surface assumed to be 300 deg. F. bofid 
line: Arbitrary straight line strength. From this the following formula is taken: Effective Structural 
Strength in Compression for each linear inch of wall equals 800 times each inch of wall thickness 
exposed to temperature of 2300 deg. F. and less. 



that will cover the most unfavor- 
able conditions ordinarily encoun- 
tered, is feasible by assuming a 
wall whose unit strength increases 
uniformly from zero on the hot 
side to a maximum on the cool 
side. Considered as a column, its 
effective moment of inertia, and 
therefore its strength, is theoreti- 
cally two-thirds of that of a wall 
in which an average strength is 



assumed to be uniformly distrib- 
uted. 

Hence, if the value found by the 
method of averages is reduced by 
one-third, a strength is determined 
that is believed to be always on 
the safe side. This will represent 
the ultimate strength, to which a 
suitable factor of safety should be 
applied. 



5 



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1600 



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Section No# 1 




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Section No. 3 



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Section No # 5 



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H Beam Relnf orclngv_ jj 



Roof Slab 




Fig. 5 Refractory concrete flat arch for precast or cast in 
place: Pour sections 1, 3 and 5; remove forms "A" (See Fig. 
pour sections 2 and 4. 



place sections 
6) ; paint 



Procedure for cast-in- 
with fire' clay slurry; 



section of arch itself is a rein- 
forced concrete beam to carry the 
weight of the arch section. The 
concrete in the flat portion of the 
section is composed of the cement 
and an aggregate of low conduc- 
tivity. The concrete in the beam 
surrounding the reinforcing bars 
is composed of the cement and an 
aggregate of higher conductivity. 
The concrete in the flat portion has 



a low heat conductivity as a result 
of the use of the insulating aggre- 
gate, while the concrete in the 
beam has a higher heat conductiv- 
ity as a result of the use of dense 
aggregates. The higher conduc- 
tivity concrete in the beam and the 
lower conductivity concrete in the 
slab each has an important bear- 
ing on the temperature of the re- 
inforcing steel. Stated otherwise, 



7 



Wood For« 




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Fig. 6 Diagratnma view of forms and .onforclnu. hao In place. This method used in con 
ructtng flat arch shown In Fig. 5 



the diffi in tin onducth I 

etes has an in. 

beaiing on 1 " .uireri 

t) s Of t) I lab to mail ■ I 

nj, in tl ing 

high i 6 )'• 

While this method ha been 
l; plied to flat r Lab 

plied to erl al > 

floor - as well. Tbl BUel hod 
limited to t*-mp< ares i 
wj h satisfa insulating a«- 

g I i tail -• fad 

coats, 1; '»f no • '« or units 

f hijihh *■ frartory mat* 

. uee'i 1 protect th< insulal rig 

cte. The fir a h or 



wail is then serviceable at mud 

high< i temp, iturcs than is the 

unproi d insulating concrete 

i • t, k. and 9 -how the thick- 
i,. • t tin- slah using in slating 

%ggr« eatei f concrete with ;i 1 

f, r of 2, 8 and 4 when u 1 
with a COIN • ' 1< in th< beam w ilh 

K f. toi in ' si< slating 

■ I the - () f i i lah u - I \y>. 

7, 8 and 9 Cor reinforced beam 
on , v i he n infoi cing 

. not n B 1 D 2" D the e>; 

f i}< l as shown 

ii I 

Y I. th< I show. K rtej 

rnurh In; than 1 f >, it 1" re 



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2500 



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lO 



(1) Reinforced Concrete 3" or 4" Thick* 
K As Shown on Graph* 

(2) Insulating Concrete, K 2. Thickness 
As Shown on Graph. 



Temperature at Joint 600 Deg. F # 



(4) Temperature of Hot Face as Shown on Graph 



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Thickness of Insulating Concrete, Inches 



7 Thickness of slab using insulating aggregates for concrete with a l< factor of 2, when 
used with a concrete in the beam with higher l< factors. 



so far been demonstrated than an 
economical concrete can be devel- 
oped which will have a much high- 
er K factor. Indications are that 
at temperatures of 600 deg. F. and 
lower, the use of olivine or magne- 
site offers the greatest promise but 
that the most that can be hoped 
for from either of these aggregates 
used in concrete is to develop a K 
factor of approximately 15. For 
concrete made of trap rock a K 
factor of 8 may be assumed, while 
with aggregate made from old fire 
brick a K factor of only 6 can 



safely be used. 

It has not been definitely proven 
that a temperature somewhat high- 
er than 600 deg. F. could not be 
used for ordinary carbon steel. 
However, it is felt that this tem- 
perature is conservative and until 
such time as experience shows it 
to be too conservative it should be 
used. 

Application to High Temperature 
Furnaces 

Refractory concrete will un- 
doubtedly be used in furnaces, the 
temperatures of which are entirely 



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Insulating Concrete 3" or 4" Thick. 
K As Shown on Graph. 

Insulating Concrete. K 4. Thickness 
As Shown on Graph. 

Temperature at Joint 600 Deg . P. 



(4) Temperature of Hot Pace A a Shown on Graph 



2 500 



. 2000 



u- 



2 

t 1500 

8 

m 

u 

I 

I 

c 

** 1000 

c 



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500 




Thickness of Inaulattn* Concr.t., Ioch«* . 



Fin 9 Thlchnui of a slab tiling nrj aggregate* for concrete with a K fictor of 4 when 

usnl with .1 concrete in the beam with higher K factor. 



..I Mr '"' P^'d in pi of the 

••Mi .1 bar supports. 

In this method conditio are 

ither ideal; whereas, when ren 

remg 1 i- embedded In the 

mcrete, the expansion of the steel 

and the shrinkage of the concrete 
whi-n exposed to elevated temper 
ires, theorvt i illy at bast, tend 
• break down the concrete. The 

method shown in Fig. to is much 

mure compatible with the volume 



change of thi wo material - when 
heated to elevated temperatun It 
noted that the shrinkage of the 
concrete, to some extent at leas 

mipensatea for the expan »n of 

the steel. 

While no installatioi D ring the 
method shown in Fig. 10 h e been 
made at the present time of * t- 
inp. 1 ... 11 shows ■ full floating 
flat arch 4 in. thick and 8 f 
square, which is the principle util- 



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ized for this method of construc- 
tion. 

Fig. 12 also illustrates a method 
of construction for precast or cast- 
in-place sections. This method of- 
fers a simple and flexible method 
which can be used for large sec- 
tions. 

Figs. 15 and 16 illustrate the 
method of reinforcing shown dia- 
grammatically in Fig. 12, except 
that there were no cross angles 
installed. 

While the information given 
relative to the amount of steel 
buried in the concrete and the 
amount exposed to the air is 
thought conservative, it should be 
borne in mind that more experi- 
ence will be needed to lay down 
any hard and fast rules. 

The question of sprung arches is 
relatively simple. They may be 
cast in place or precast in sections 
and lifted into place with a crane. 
Sections of arches with a 14'0" 
span, weighing 3 tons with no 
reinforcing, have been precast and 
set in place with a crane. Where 
doubt exists about the strength of 
the concrete in an arch section it 
can be reinforced with angle irons 
on the outside corners. When so 
used one flange of the angle is ex- 
posed to the air for rapid dissipa- 
tion of the heat to maintain a low 
temperature in the steel. The an- 
les must be bent to conform to 
the radius of the arch and fas- 
tened together at the ends and 
several points in the middle. 

With reference to Figs. 13 and 
16, this method may also be ap- 
plied to flat arches and wall sec- 
tions. 

It is doubtful if in many cases 
it will be desirable to reinforce the 




Fig. 11 No installations have been made using 
the reinforcing method similar to that shown in 
Fig. 10. The above illustration shows a full float- 
ing flat arch 4 in. thick and S ft. square, which 
is the principle utilized for this method of con- 
struction. 

vertical or side walls of a furnace. 
However, in Fig. 14 reinforced 
side walls are shown. (See also 
Fig. 17). One side shows bars 
embedded in the concrete, while 
the other shows angle irons with 
one flange embedded in the con- 
crete and the other flange exposed 
to the air to facilitate the dissipa- 
tion of heat and maintain a low 
temperature in the angle. It is 
obvious that the methods of con- 
struction can be interchanged to a 
considerable extent. For example, 
the methods shown for reinforcing 
the side walls can be used for the 
bottom. It is important to remem- 
ber that the temperature of the 
steel should not exceed 600 deg. F. 
in any case. The bottom in this 
sketch shows a method of construc- 
tion similar to that shown in the 
roof arch, Figs. 5 and 6. The roof 
shows a method of construction 
similar to that shown in Fig. 10. 



13 



Flange of Angle In Concrete Alternately Bent 
In Opposite Directions As Shown 



A 




Outside Angle Cupped In Slightly 

\ 




Section "AA" 
Fig. 12 Section with angle iron reinforcing suitable for arches, waJIs and bottoms. 



While the side walls show the 
reinforcing in one direction it can 
be used in the opposite or both 
directions if desired or supple- 
mented with wire mesh as shown. 

When refractory concrete is de- 
pendent wholly upon its own 
strength it would appear logical to 
limit the dimension of any slab to 
not more than 4'-0" in any one 
direction. However, when the re- 
fractory concrete slab is reinforced 
by other methods it is felt the size 



of the slab can be increased to any 
-ize which the particular method 
will permit. 

Fig, 10 illustrates a method in 
which the maximum dimension 
should be limited. Figs. 5 and 6 
illustrate a method in which the 
maximum dimensions of the slab 
would be largely dependent upon 
the strength of the reinforced 
beam. The side walls in Fig. 14 
illustrate a method in which it is 
felt the dimensions of the concrete 



14 



might be very large — in fact, with 
almost no limit. This is also true 
of the method shown in Fig. 12. 

Repairs 

While the method of construc- 
tion is important there are under 
certain conditions other considera- 
tions of equal importance. Under 
severe conditions where repairs are 
frequent and inevitable, the means 
for making replacements is of 
equal if not greater importance. 
No great difficulty is encountered 
in repairing the bottom of most 
furnaces. After the slag is chipped 
off, another layer of refractory 

be placed on top of 
of the concrete slab 



concrete can 
that portion 



remaining. 

Much the same condition exists 
with relation to the side walls ex- 



cept that forms are necessary to 
form the inside surface while the 
remaining concrete acts as the out- 
side form. In large furnaces the 
side walls may be repaired by 
shooting a refractory concrete 
mortar with a cement gun. 

Roof arches offer a greater prob- 
lem and it is felt the method shown 
in Fig. 10 offers the simplest meth- 
od of repair especially if precast 
units are used in the original con- 
struction. For example, if slab 
No. 2 in Section No. 3 needed re- 
placement it would only be neces- 
sary to remove slabs No. 1 and 2, 
insert a new slab for No. 2 and 
replace slab No. 1. This repair 
could in many cases be made with- 
out serious interference with op- 
eration and without materially 



Cross Ties As Heeded 



3" Angle Iron 




Section "AA n 



3" Angle 




See End View 



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/-?* 



^k 






i/e n 



-T_l. 



3" Angle 

— -h- 



Enlarged Detail of Corner 



Corners of Angles 
Mltered and Welded 



End View 



Fig, 13 Sprung arch section showing angle iron frame 



15 



"A" = Thickness of Roof Slab 
"B" & "C» = i "A" 



Joint 






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300 

Dee. 




Joint 










Angles 



Reinforcing Bars 



2100 Dee. F. 



Mesh Reinforcing 



Joint 



3 r ^ - 




Mesh 
Rein- 
forcing 



Joint 








- : . s: Reinforcing Bars 



Fig. 14 Illustrating several different methods of reinforcing refractory concrete. A different n»e th °d 

of reinforcing is shown in each side. Where metal reinforcing is embedded in the concrete, it should 

be so placed that the temperature of the steel will not exceed 600 deg. F. All joints should be 

given a paint coat of fire clay slurry. 



dropping the temperature in the 
furnace. The application of the 
fire clay slurry to these joints dur- 
ing construction has an important 
part in the removal of these sec- 
tion.-. If a highly refractory clay 
is used, a plane of weakness at 
each joint will exist which will 
greatly facilitate the removal of 
i dabs. 

A cement gun can also be used 
for repairing these slabs but it 
would prevent the removal of any 
of these slabs for replacement at a 
later date. The repair of a roof 
slab as shown in Figs. 5 and 6 



would require the use of a o-ment 
gun or the removal of a complete 
section. Under these conditions it 
is felt this method of construction 
is best adapted to furnaces which 
seldom require repairs or replace 
ment. 

Where conditions are such that 
it is desirable to use reinforcing 
which will attain a temperature 
higher than the 600 deg. F. maxi- 
mum for carbon steel, special 
steels should be used. 

Size Limitations 

It is commonly accepted that 
.rick walls of 4W thickness ar 



10 




Fig. 15 Example of the method of reinforcing shown in Fig. 12 except there were no cross angles. 



not suitable for heights greater 
than about 3 feet, while 9" brick 
walls should be limited to a height 
of 8 feet; 13 M$" brick walls to a 
height of 12 feet, and for greater 
heights the brick walls should be 
18" or more thick. 

Refractory concrete can be rein- 
forced so that practically any 
thickness can be used in walls of 
almost any height. Assuming that 
the concrete wall is 80% as thick 
as the brick wall, approximately 
the same heat loss will result, 
while there will be much less heat 



storage in the concrete. This is 
explained by the fact that if clay 
fire brick are crushed to granular 
sizes, mixed with the cement and 
cast as concrete, about 15% great- 
er porosity exists in the concrete 
than in the brick before being 
crushed. This increase in porosity 
results in greater insulating value 
and decreases the weight per cubic 
foot which, when combined with 
the reduction in thickness for the 
concrete wall, makes its weight 
only about 67% that of brick wall 
of the same furnace. 




Fig. 16 Another example of the method of reinforcing shown in Fig. 12 except there were no 
cross angles. 



17 



I 



Reprinted from 
Industrial Heatin 



1940 





















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